1. Field of the Invention
The present invention relates to a line protection switch equipment in a communication system, for switching from a working line (i.e., a normal line) to a protection line (i.e., a standby line) to continue communication when a fault occurs on the working line.
2. Description of the Related Art
To improve the reliability of a communication system, a one to one or n to one working-protection switching system is employed for continuing the communication by separating the faulty portion and by switching to a protection equipment or to a protection line. In such an n to one working-protection switching system, the switching circuit for switching and connecting the faulty line among the working n lines and one protection line must be maintained in the switching state even when the power is cut, and should have a simple construction.
Active elements such as transistors or amplifiers cannot be employed in the switching circuit to maintain it in the switching state during a power cut off. Also, to keep the construction of the switching circuit as simple as possible, the switching circuit must not be formed by a matrix switch.
An example of a prior art reference is FUJITSU Sci. Tech. J. 21, 1 (March 1985), pp 50-52 "405 M bit/s Single-Mode Optical Fiber Transmission System".
In an n to one working-protection switching system in the prior art having n working lines and one protection line for switching and connecting the protection line in place of the faulty working line, the protection line is previously determined. The switching and connection are carried out by changing the faulty working line to a protection line and by making the protection line a working line. When the fault is recovered, the working line which became the protection line is switched to again become a working line, and the protection line which became the working line is switched to again become a protection line.
In the afore-mentioned prior communication system, the protection line is previously determined. Therefore, a disadvantage arises in that there is little freedom of line selection. To remove this disadvantage, a floating (or quasi-fixed) system is known. In the floating protection system, the working-protection switching can be carried out without specifically determining a protection line so that the freedom of line selection is increased. The switching circuit, however, has a matrix switch construction. Further, since a high speed data switching of several ten to several hundred mega bits/s must be effected in accordance with the degree of multiplication, the switching circuit cannot be realized by a simple contact construction. Therefore, the circuit having a matrix switch construction has a disadvantage of being large in scale.